The filoviruses Marburg virus (MARV) and Ebola virus (EBOV) are negative-sense RNA viral pathogens that cause periodic outbreaks of severe disease in humans. Despite their importance as emerging pathogens and as public health threats, many aspects of filovirus biology remain understudied. One major knowledge gap concerns the function(s) of the non-protein coding sequences present in filovirus genomes that correspond to 5? and 3? untranslated regions (UTRs) in viral mRNAs. These sequences comprise ~22% of the MARV genome and a similar amount of the EBOV genome and yet have been barely studied. Another knowledge gap concerns mechanisms by which viral mRNAs are translated to protein and how this may be sustained when innate immune responses are activated. Beginning to address this, we previously demonstrated a role for an upstream open reading frame (uORF), present in the 5?UTR of the EBOV L open reading frame (ORF), in the regulation of L translation. We further implicated this uORF as providing a means to sustain L expression under conditions innate antiviral defenses are activated. These data support a role for the UTRs as translational regulators which can modulate viral gene expression in the face of innate antiviral defenses. Our recent profiling of MARV 3?UTR function suggests negative-regulatory elements are present in the nucleoprotein (NP) mRNA and that positive regulatory elements are present in the L mRNA. In addition, we previously reported that the MARV genome appears to be edited by adenosine deaminase activity, particularly in sequences corresponding to the nucleoprotein (NP) mRNA 3?UTR. We now show that such mutations substantially increase mRNA translation efficiency, suggesting a means to maintain NP expression when the IFN-induced p150 form of adenosine deaminase acting on RNA 1 (ADAR1) is upregulated. Interestingly, the regulatory elements within the wildtype NP 3?UTR are also capable of triggering innate immune responses, but this immune stimulating activity is alleviated by the presumptive ADAR1 editing mutations. To follow up on these observations, we propose a combination of transfection- and live virus-based experiments to define the translation regulatory elements in MARV and EBOV 3? UTRs and to determine how ADAR1 editing impacts viral gene expression and virus replication. We will fully evaluate the capacity of MARV and EBOV 5? and 3?UTRs to trigger innate antiviral responses, defining the sensors responsible for this induction. Finally, we will create recombinant MARVs with wildtype and mutant 3?UTR sequences. These will be tested in wildtype and ADAR1-deficient cells and mice in order to determine how translational regulatory elements and ADAR1 editing impact viral growth and pathogenesis. Cumulatively, these studies will provide significant new insight into filovirus-host interactions related to translation and innate immune evasion.